In recent years, development of personal authentication techniques by a wireless communication system has been promoted for securing an information security. In concrete, with wireless communication equipment carried by a user and wireless communication equipment provided for a physical object such as a personal computer, a portable telephone, a vehicle or the like, authentication is consistently performed by the wireless communication systems. When the physical object enters a certain range of peripheries of the user, control of the physical object is enabled. When the physical object goes out of the certain range of peripheries of the user, control of the physical object is disabled. In order to judge whether or not the physical object exists within the certain range of peripheries of the user, it is necessary to measure a distance between the physical object and the user by a wireless communication apparatus at the time of wireless authentication communication.
Moreover, there is measurement by received field intensity as a simplest distance measurement method. No specific circuit is necessary for the distance measurement, and the distance can be measured by utilizing wireless communication equipment for wireless authentication. However, since the user carries the wireless communication apparatus or an authentication key device, the gain of the mounted antenna is strongly influenced by conductors such as the human body. Moreover, when it is used in a multipath environment, the antenna suffers an influence of fading.
For the above reasons, a phenomenon that the received field intensity rapidly decreases due to the surrounding environment occurs. Consequently, a relation between the distance and the received field intensity such that the received field intensity decreases as the distance increases collapses, and distance measurement accuracy largely deteriorates. Moreover, the antenna gain falls below the necessary antenna gain during the authentication communication, and this incurs a decrease in the communication quality. Conventionally, a method for using a small loop antenna having a structure such that, even if a conductor is located adjacent to the antenna, a loop plane is perpendicular to the conductor is proposed as a method for avoiding the influence of the conductor on the antenna in order to prevent the rapid decrease in the gain (See, for example, FIG. 1 of Patent Document 1 and FIG. 2 of Patent Document 2). Moreover, a method for radiating a different polarized wave component has been proposed as a method for preventing the influence of fading (See, for example, FIG. 4 of Patent Document 1).    Patent Document 1: Japanese patent laid-open publication No. JP 2000-244219 A.    Patent Document 2: Japanese patent laid-open publication No. JP 2005-109609 A.    Patent Document 3: International Publication WO2004/070879.    Non-Patent Document 1: Editor of The Institute of Electronics, Information and Communication Engineers, “Antenna Engineering Handbook”, pp. 59-63, Ohmsha, Ltd., First Edition, as issued on Oct. 30, 1980.